I think the effects of blackholes on the surrounding space is localised to a small area around the blackhole but I wondered if the largest blackholes at the centre of massive clusters could be a reason for the expansion of space?

If at the event horizon, space-time is compressed could this have a stretching effect on the space-time fabric within the voids between clusters? Again I dont really think this is the reason for the expansion rate to be increasing over time but as these central blackholes become larger and their effect on their local space-time becomes greater, it made me wonder if this could be what is stretching out the space between clusters giving the expansion of the universe?

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    $\begingroup$ I am new here but I know that a down vote deserves an explanation, otherwise I would assume someone with no knowledge decided to down vote,, btw votes are no concern to me, answers and correct comments are what is important. $\endgroup$ – Nathan O'Haire Nov 29 '20 at 18:26
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    $\begingroup$ otherwise I would assume someone with no knowledge decided to down vote That's not a good attitude. Someone will typically downvote in this way either to flag annoyance at what the consider a very bad question and/or because they don't like the question but have not yet acquired enough reputation to downvote. My impression is that you have some misconceptions about spacetime ("fabric" is a common sign of this) and that view is perhaps shared by who ever downvoted. BTW, no one is required to explain their reason for downvoting - they're entitled to remain anonymous, just as upvoters are. $\endgroup$ – StephenG Nov 29 '20 at 23:54
  • $\begingroup$ Well here is some more bad attitude for you Mr G, get ready with your down vote, i am really not interested in your opinion. $\endgroup$ – Nathan O'Haire Nov 30 '20 at 1:07
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    $\begingroup$ Someone had already answered your question so it would be redundant for me to do it - redundant answers are discouraged on SE generally. As for your claim that "you don't care", you posted two comments moaning about my one comment, so, yes, actually you do care. Get used to reading comments you don't like - it's the nature of a public area like this that people will post things you do not like or agree with. Taking the position that everyone who votes your question down or disagrees with you "has no knowledge" is just a very bad attitude. $\endgroup$ – StephenG Nov 30 '20 at 7:38
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    $\begingroup$ @NathanO'Haire please stay friendly. Just because StephenG (friendly) criticized your question, this is no reason to become insulting $\endgroup$ – Jonas Nov 30 '20 at 16:53

That would be a reasonable hypothesis if spacetime really were some kind of fabric. Unfortunately, the fabric analogy is fairly limited, and taking it too literally can lead to misconceptions.

Dark energy doesn't cause spacetime expansion, it causes the expansion to accelerate. Until the late 1990s, we expected the expansion to be decelerating due to the effects of gravity. It was rather surprising to learn that it's actually accelerating.

We still don't know exactly why it's accelerating, but we do know that the acceleration is uniform on the very large scale. Expansion itself isn't significant on scales smaller than the scale of galaxy clusters. It's so weak that it's overwhelmed by gravity (which is itself weak relative to the electromagnetic and nuclear forces) at smaller scales.

But at the very large scale that cosmologists focus on, where galaxies are treated like dust particles, the acceleration of expansion is fairly independent of gravity. If your theory were correct, we'd expect to see more acceleration in the huge voids between galaxies, but that's not the case. So although we don't know much about dark energy, we do know that its distribution is fairly uniform.

  • $\begingroup$ What is your definition of "very large"? within a few hundred million light years expansion is much faster than at billions of light years and that is still not constant. Dark energy is expansion, without either gravity would take over. I dont understand why you consider them different phenomenon. The rate of expansion is not uniform, recent papers have revealed that, whether it is a mistake in ours ways of measuring is another issue. $\endgroup$ – Nathan O'Haire Nov 29 '20 at 20:37
  • $\begingroup$ @Nathan Like I said, "where galaxies are treated like dust (or fluid) particles", so billions of light years. The equations of GR allow for expansion (or contraction) without requiring dark energy. In fact, it's rather unlikely to have a static universe that isn't either expanding or contracting. However, the fact that the expansion is accelerating doesn't naturally fall out of the equations, which is why we had to postulate dark energy. There's some info about it here: physics.stackexchange.com/a/261234/123208 and I guess Pela has posted about it on this site too. $\endgroup$ – PM 2Ring Nov 29 '20 at 21:01
  • $\begingroup$ Also see the classic paper by Davis & Lineweaver: Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe &/or the simplified version (with more diagrams) they wrote for Scientific American, here. $\endgroup$ – PM 2Ring Nov 29 '20 at 21:06
  • $\begingroup$ I understand how expansion works over further distances and it only seems like things are expanding faster close to us because the light that reaches us is more recent but I still dont understand how expansion is different from dark energy and I dont mean expansion due to clusters drawing matter which does make expansion faster on opposite sides of voids. $\endgroup$ – Nathan O'Haire Nov 29 '20 at 22:02

It's an interesting thought experiment but there are a few problems.

According to general relativity it doesn't matter the compactness of the central body. Objects orbit a black hole the same way they orbit an equal mass star provided the distances are the same. From a distance there should be no change as the central body gets more compact.

General relativity correctly predicted gravity waves, light curvature around the sun, the Schwarzschild radius of black holes, gravitational time dilation and Mercury's orbital hick-up, so it scores a lot of points for making correct predictions and general relativity says your argument isn't how spacetime works. Granted, general relativity doesn't explain dark energy but so far, it explains gravity very well.

Stretching and expending space the way you suggest would imply that gravity creates more space between objects, which is not what we observe. To put that another way, if it creates dark energy expansion between galaxies, who wouldn't it create expansion of objects orbiting much closer to the black hole?

Granted, we've never observed orbits around black hole up close, but your theory goes against both the consensus and observations. Orbits appear to follow Einstein's and Newton's laws pretty much exactly. There's evidence of gravitational perturbations (Mercury's orbit) but no evidence of gravitational stretching space to create greater distances between objects.

A 2nd problem is that dark energy seems too be evenly spread not bunched around galaxies. The black holes in the observable universe are somewhat structured by into clusters and superclusters. Dark energy doesn't appear to share that kind of cluster structure. The WFIRST Telescope, due to launch in 2024 should get a better image of dark energy, do it might shed some light on this.

A 3rd problem is that dark energy is the most abundant thing in the universe. Black holes, even supermassive ones, are just a small percentage of galactic mass, so it's difficult to explain why the relatively local and comparatively small mass of black holes would generate so much dark energy, greater than all the mass of all the galaxies and dark energy combined.

That's just my take. Feel free to read it with a grain of salt.

Could gravity drag or stretch space? Well, maybe yes. Gravity probe B found that rotating massive bodies drag space around with them, so maybe space can be stretched a little. This is called frame-dragging.

But these effects of stretched space, lets include general relativity predicted gravity waves and frame dragging are very small and very local. It's hard to explain why they would affect the expansion of space across millions and billions of light years.

A final problem that I feel is worth mentioning, is that physicists don't know what gravity is. They know how it behaves, but only have untestible hypothesis of what it is. They think it's probably a very very low mass, very hard to detect, spin two particle called a graviton, that's so weak that it may be impossible to detect.

Physicists also don't know what dark energy is, so, if your theory is that this one thing - which physicists don't understand, is tied to this 2nd thing - which physicists don't understand. That's going down the rabbit hole of unpredictability and un-testability pretty far.

Many of the ideas in quantum physics are successful because they've made incredibly accurate predictions. The ideas may seem highly counter intuitive but the predictability makes a lot of it a valid theory. We have no theory of gravity and no theory of dark energy, just ideas that may or may not be accurate.

Good physics requires testability and repeatability, or at minimum, a very convincing set of equations. Gravity and Dark energy are on the outskirts of that, in the realm of the unknown.

Please don't mistake my "this problem/that problem" answer as dismissal, because I think yours is a cool idea that a black hole could stretch space and maybe it can, but it's a tough sell, because dark energy appears to be everywhere, and there's so much of it and there probably weren't always black holes, they presumably came later, after the first stars, which came well after the cosmic microwave background radiation, which came well after expansion. Just my 2 cents.

  • $\begingroup$ Gravity compresses space which is what draws things into its path of concentric rings of space-time, this is seen in the multiple rings of an accretion disk, it wouldn't create expansion with gravitationally bound objects but all galaxies that are not bound are moving away even if they are still flowing to a cluster.. anyways like you say its hard without more evidence to prove it. thanks for taking the time to answer. $\endgroup$ – Nathan O'Haire Nov 30 '20 at 15:37

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